In the optical communications space, various techniques are used to synthesize an optical communications signal for transmission. A popular technique utilizes a laser 2 coupled to an external optical modulator 4, as shown in FIG. 1a. The laser 2 generates a narrow-band continuous wave (CW) optical carrier signal 6 having a desired wavelength. The optical modulator 4 operates to modulate the amplitude and/or phase of the carrier signal 6 to generate the optical communications signal 8 based on a drive signal 10 that encodes data to be transmitted. Typically, the drive signal 10 is generated by a driver circuit 12 based on an input data signal x(t).
In the arrangement illustrated in the FIG. 1a, the optical modulator 4 is provided by a well known Mach-Zehnder (MZ) interferometer. Other types of modulators may be used, depending on the desired type of modulation. For example, an electro-absorptive modulator (EAM) or a variable optical attenuator (VOA) may be used for amplitude modulation; whereas phase shifters are well known for implementing phase modulation. In each case, the driver circuit 12 generates the drive signal 10 by scaling the input data signal x(t) to satisfy the voltage and current requirements of the modulator 4. The driver circuit 12 may also generate one or more bias signals (not shown) for controlling a bias point of the modulator 4 in a manner well known in the art.
An alternative approach is to directly modulate the laser itself. As is well known in the art, a conventional semiconductor laser exhibits responses, in both amplitude and frequency (wavelength), to an input drive current. Both of these responses can be modeled using respective transfer functions. The paper “Shaping Current Waveforms for Direct Modulation of Semiconductor Lasers”, Illing et al., Institute for NonLinear Science, U.C. San Diego, 2003, describes a method and system for directly driving a laser to generate an amplitude modulated optical signal. In this case, the problem is to define a drive current which causes the laser output to “cleanly” transition between high and low output levels (respectively representing binary ‘0’ and ‘1’). This is achieved by deriving a model (transfer function) of the amplitude response of the laser, which accounts for delayed effects of population inversion. The inverse of the transfer function can then be applied to the input signal x(t) to obtain a drive current I(t), using a driver 14, which will produce the desired laser output, as may be seen in FIG. 1b. 
A limitation of this approach is that it considers only the amplitude response of the laser. As is known in the art, semiconductor lasers exhibit chirp, which is a variation in the output wavelength (frequency) with the drive current level. As a result, modulation of the drive current produces corresponding variations in the output wavelength, in addition to the desired amplitude modulation.
Chirp is a product of laser carrier dynamics, principally the resultant effective refractive index of the laser's gain region as a function of carrier density. Due to the interaction between the optical signal within the laser cavity and optical gain (due to the population inversion associated with injected carriers) the optical signal is frequency modulated as the electrical drive current is modulated. In the presence of non-zero optical fiber dispersion, chirp of a directly modulated laser causes inter-symbol interference (ISI) that is a function of propagation distance. This problem means that the signal reach is not just a function of the laser's amplitude fidelity but also the accompanying chirp. The greater the chirp, the greater the ISI for a given propagation distance.
An additional limitation of the Illing et al. paper is that it only provides a method of transitioning the laser output between a pair of stable amplitude values (i.e. binary ‘0’ and ‘1’). As may be appreciated, this requires a bi-stable response with an acceptable noise figure. However, in some cases, it is necessary to obtain a fully linear response. For example, Applicant's co-pending U.S. patent application Ser. Nos. 10/262,944, filed Oct. 3, 2002; 10/307,466 filed Dec. 2, 2002; and 10/405,236 filed Apr. 3, 2003; and International Patent Application No. PCT/CA03/01044 filed Jul. 11, 2003 describe techniques for compensating impairments in an optical link by predistoring an input signal, in the electrical domain, and then using the thus predistorted signal to drive optical modulation. As described in those applications, successful implementation of this technique, particularly for the case of polarisation dependent and non-linear impairments, requires continuous (that is, analog) modulation of both the amplitude and phase of a CW optical carrier. Stated more generally, it is desirable to arbitrarily modulate the E-field of the CW carrier, within the complex plane.
As mentioned in the above-referenced co-pending applications, one method of accomplishing this result is to use a directly driven laser cascaded with an amplitude modulator (e.g. a conventional Mach-Zehnder interferometer), as shown in FIG. 2. This approach exploits chirp to produce a phase-modulated optical carrier, which can then be amplitude modulated by the MZ modulator. In this case, the driver circuit must generate a pair of drive signals in the form of amplitude and phase (or, equivalently, frequency) signal components VS(t) and VP(t). The amplitude component VS(t) drives the amplitude modulator 4 to modulate the amplitude of the CW carrier signal 6 in a conventional manner. The frequency component VP(t) provides the laser drive current, and is varied to induce desired excursions of the laser output phase.
In principle, this technique enables E-field modulation of the CW carrier within the complex polar-coordinate (Amplitude-Phase) plane, limited primarily by the frequency response of the laser 2. However, actually achieving this result requires that the amplitude and phase must be independently controlled. Since a semiconductor laser exhibits both chirp and amplitude modulation in response to drive current modulation, it is not possible to decouple the phase and amplitude modulation of the laser output.
Accordingly, methods and apparatus for cost-effectively synthesizing a modulated optical signal using a directly driven laser remains highly desirable.